Estimation of the weighted CTDI(∞) for multislice CT examinations

The aim of this study was to examine the variations of CT dose index (CTDI) efficiencies, ε(CTDI(100))=CTDI(100)/CTDI(∞), with bowtie filters and CT scanner types.

This was an extension of our previous study [Li, Zhang, and Liu, Phys. Med. Biol. 56, 5789-5803 (2011)]. A validated Monte Carlo program was used to calculate ε(CTDI(100)) on a Siemens Somatom Definition scanner. The ε(CTDI(100)) dependencies on tube voltages and beam widths were tested in previous studies. The influences of different bowtie filters and CT scanner types were examined in this work. The authors tested the variations of ε(CTDI(100)) with bowtie filters on the Siemens Definition scanner. The authors also analyzed the published CTDI measurements of four independent studies on five scanners of four models from three manufacturers.

On the Siemens Definition scanner, the difference in ε(CTDI(W)) between using the head and body bowtie filters was 2.5% (maximum) in the CT scans of the 32-cm phantom, and 1.7% (maximum) in the CT scans of the 16-cm phantom. Compared with CTDI(W), the weighted CTDI(∞) increased by 30.5% (on average) in the 32-cm phantom, and by 20.0% (on average) in the 16-cm phantom. These results were approximately the same for 80-140 kV and 1-40 mm beam widths (4.2% maximum deviation). The differences in ε(CTDI(100)) between the simulations and the direct measurements of four previous studies were 1.3%-5.0% at the center∕periphery of the 16-cm∕32-cm phantom (on average).

Compared with CTDI(vol), the equilibrium dose for large scan lengths is 30.5% higher in the 32-cm phantom, and is 20.0% higher in the 16-cm phantom. The relative increases are practically independent of tube voltages (80-140 kV), beam widths (up to 4 cm), and the CT scanners covered in this study.